Many systems and devices use communications based on Bluetooth (BT) standards for communicating with nearby BT-enabled devices. For example, a computer can perform BT communications with portable communication devices, such as one or more tablets, smartphones or smartwatches, or with one or more printers, speakers, sensors or other BT-enabled devices. BT standard (e.g., BT 5.0) introduces a new feature called stable modulation index that applies to BT low-energy (BTLE) modulations such as LE-1 Mbps (LE-1M), LE-2 Mbps (LE-2M), and LE long range (LELR). A transmitting LE device that supports stable modulation index may inform a receiving LE device of this fact. The receiving LE device may benefit from this feature and improve its sensitivity by about 3 dB to 4 dB by using coherent receivers.
On the transmitter side, the difference between standard modulation index and stable modulation index is based on a modulation characteristic parameter called Δflavg. The parameter Δflavg is a measurement of the average frequency deviation when transmitting a bit sequence consisting of 11110000. The requirement for Δflavg parameter is less restricted for standard modulation index than for stable modulation index. For example, for LE-1M and LELR, the range of values for the parameter Δflavg is between 225 KHz and 275 KHz, for standard modulation index, and between 247.6 KHz and 252.5 KHz, for stable modulation index. Similarly, for LE-2M, the range of values for the parameter Δflavg is between 450 KHz and 550 KHz, for standard modulation index, and between 495 KHz and 505 KHz, for stable modulation index. LE receivers can benefit from stable modulation transmitters by leveraging a coherent detection scheme or a maximum-likelihood sequence estimation (MLSE) demodulator. These demodulators can provide about 3 to 4 dB improvement in sensitivity relative to a standard non-coherent Gaussian frequency-shift keying (GFSK) demodulator.
Typically, MLSE or coherent demodulation schemes cannot be employed for standard modulation index, as they may not work properly when an LE transmitter uses a dirty transmit (TX). This may prevent the device from passing bench qualification. A substantial improvement in sensitivity with stable modulation index can be expected. Improvements of between 0.5 dB and 4 dB have been demonstrated for all LE rates, including LE-1M, LE2M, LELR-125 Kbps and LELR-500 Kbps.